The overall constitution of a mobile radio communications system will be explained with reference to FIG. 9, which shows the overall constitution of both this invention and the prior art. In a mobile radio communications system, mobile stations 100.sub.1 -100.sub.4 and radio base stations 101.sub.1 -101.sub.k are connected by radio channels, and radio base stations 101.sub.1 -101.sub.k and mobile switching center 107 are connected by wire circuits 106.sub.1 -106.sub.k. Radio base stations 101.sub.1 -101.sub.k and mobile switching center 107 communicate via wire circuits 106.sub.1 -106.sub.k by means of fixed-length packets.
Asynchronous Transfer Mode (ATM) is a communications technique which uses fixed-length packets and which is being considered for use in fixed networks. The constitution or make-up of the signals used in ATM transmission systems will be explained with reference to FIGS. 10(a) and 10(b), which shows the constitution of such signals. There are two methods for making low bit-rate signals into cells in an ATM transmission system. In the first method, as shown in FIG. 10(a), data is accumulated until it fills one cell. In the second method, as shown in FIG. 10(b), an ATM cell is formed by accumulating data in a cell during an allowable cell-creation delay time, up to which time the quality of the information signal will not deteriorate due to the delay involved in making the cell. Thereafter, the portion of the cell length that is not filled is left empty of data and is filled with dummy data.
Suppose that the method illustrated in FIG. 10(a) is applied to ATM transmission in a fixed network in which data is transmitted by fixed-length packets. If information signals are transmitted at 64 kbps, then because the delay time involved in making a cell is only 6 ms, there is little deterioration in signal quality.
However, in mobile radio communications systems, given the need to make effective use of frequency resources in the radio section, information signals are converted to low bit-rate signals using data compression techniques. If it is assumed that the coded and compressed information signals in a mobile radio communications system are 8 kbps signals, then, if an ATM cell is completely filled with such signals, the delay time involved in making a cell will be 48 ms.
Deterioration of signal quality due to echoes and the like can no longer be ignored when delay time increases in this way. Now, on the assumption that information signals in a mobile communications network are, as previously described, 8 kbps signals, and that the intention is to make cells with the same cell-creation delay time as the 64 kbps information signals that are transmitted for example in a fixed network, then the amount of information held in a cell will be 8000.times.0.006=48, i.e., 48 bits or 6 bytes. Because a 48-byte data section is provided in a single ATM cell, the utilization efficiency of the data section under these circumstances will be only 12.5%, which is very low (see FIG. 10(b)), and the end result is a low throughput network. The shortcoming of this system is that the poor information throughput in the network means that it is difficult to construct an economical mobile communications network.
The present invention has been devised against this background, and it provides an economical mobile radio communications system which avoids deterioration in information signal quality and makes effective use of transmission lines.